Cities were already known to retain more heat than the rural environments that surround them, but new modeling from researchers in the United Kingdom now suggests that urban areas are also more sensitive to changes in climate. Furthermore, they will experience greater increases in average temperature with rises in atmospheric carbon dioxide, and the cooling effects of night will become more of a memory than a reality.

Meanwhile, Washington, D.C. -- where Congress is debating over whether to pass a climate bill -- is getting a memorable preview of what new computer models are predicting. Last week's temperatures broke a 100-year record, and forecasters expect this June will be the hottest ever recorded in the area.

Urban areas produce their own environments. Vegetation is replaced with steel and Concrete. A natural breeze is reconfigured by skyscrapers and other tall buildings. Soil is covered with black asphalt and loaded with automobiles.

Each transformation contributes something more to what is known as the urban heat island, or UHI, effect, a phenomenon scientists have known about for almost 200 years.

Instead of being consumed by plants or transported away by soil moisture, much of the daytime heat directed into urban areas is absorbed by hard, impermeable surfaces that have no other way of releasing their stored heat except to re-radiate it at night. This gives residents of urban areas little relief from the summer heat long after the sun sets.

In New York City, evening air temperatures can be up to 14 degrees Fahrenheit warmer than those of rural areas that lie within roughly 60 miles, according to a study published by American Meteorological Society in 2009.

Feeling a rise of 5 degrees by 2050
This stored heat will only get worse with increased atmospheric carbon dioxide, said Mark McCarthy, lead author and a research scientist with the Climate Impacts team at the Met Office, the United Kingdom's weather service. The analysis, published in Geophysical Research Letters, was co-authored with Met Office colleagues Richard Betts, head of Climate Impacts, and Martin Best.

The research found that "urban areas are warming faster" than rural ones, in response to rising levels of carbon dioxide, said McCarthy. Their models predict that urban daytime temperatures will rise by more than 5 degrees Fahrenheit in most parts of the world when carbon dioxide levels in the atmosphere reach 645 parts per million, a figure possible as early as 2050.

Nighttime temperatures will also rise by similar numbers. However, in the Middle East, where the UHI effect is the most extreme, cities throughout the region could feel an additional 5 degrees at night.

Besides the Middle East, some of the regions whose local climate is most sensitive to urbanization, including central Asia and western Africa, are also expected to double or even triple in population by 2050, according to U.N. estimates. By that time, more than 68 percent of the world's population will reside within urban areas -- up from 50 percent in 2009.

These trends in temperature and population migration, McCarthy said, are sure to have significant consequences for human health by raising the possibility of heat-related fatalities. According to the U.S. Global Change Research Program, winter cold snaps raise death rates by 1.6 percent, while heat waves are far more lethal, pushing death rates up by 5.7 percent.

Among the health risks associated with heat waves are heat exhaustion and heat stroke. Symptoms can include vomiting, diarrhea and fatigue. The most vulnerable include infants and children, pregnant women and elderly people with chronic medical conditions. Outdoor workers and people living in poverty are also at risk.

In modeling the properties of urban environments, the Climate Impacts group also found that the number of hot nights in most cities, worldwide, is expected to rise significantly.

Complex environments that are hard to model
In a world with carbon dioxide concentrations of 645 ppm, cities such as Los Angeles; Tehran, Iran; and Delhi, India, will experience about three times as many hot nights than in one with half as much carbon dioxide.

Despite the seriousness of these predictions, previous studies have chosen not to incorporate urban environments for a variety of reasons, McCarthy said.

One reason is that urban heat island research has largely grown up among a separate camp of climatologists from those who designed the earliest climate models. There have also been exclusions because of the resolution limits of the most modeling techniques. Another reason, and possibly the simplest, is that cities are extremely complex environments, and it is difficult to know how to incorporate them into any model.

How does one evaluate the amount of wind a group of tall buildings can channel or how much heat they retain against the considerable shade they can provide? How much heat is transferred to pavement and buildings by the presence of human beings? There are still many more questions than there are answers in this field, which continues to make urban areas a major stumbling block for climate modelers.

Gaffin, who has studied climate change since the mid-1980s and UHIs for the last seven years, said he was "thrilled" to see scientists looking to integrate urban environments into climate models. In Gaffin's opinion, "fighting urban heat islands is a proxy for fighting global climate change."

A co-author on the 2009 American Meteorological Society temperature study of New York City, Gaffin has worked with Mayor Michael Bloomberg's administration to develop UHI mitigation projects to reduce the city's heat stress.

One such project is called the "NYC Cool Roofs" program, an initiative to reduce electricity use -- and thus greenhouse gas emissions -- by coating rooftops with reflective, white paint, which can reduce indoor temperatures by 10 to 20 degrees, according to the mayor. But, Gaffin said, New York is not alone in this effort, as many cities around the world are starting to handle what he called their "sunlight management."

"Every degree is huge, in a city," Gaffin said. "It's the difference between a blackout and getting through a heat wave." And a blackout can spell life or death for more vulnerable members of a community.

In fact, peak urban electricity demand increases roughly 1.5 to 2 percent for every 1 degree Fahrenheit that exceeds about 68 to 77 degrees, according to a 2005 study by Hashem Akbari, head of the Heat Island Group at Lawrence Berkeley National Laboratory.

Last week, it was announced that the Richard Betts and the Climate Impacts team at the Met Office had been selected to be among the scientists to contribute to the next climate report.

"There was a recognition that urban climate models were missing in the fourth report," said McCarthy, whose work will be part of that update. The Intergovernmental Panel on Climate Change's Fifth Assessment Report is scheduled to be published between 2013 and 2014.

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